Rising dominance means Bitcoin is outperforming altcoins or capital is fleeing to it in a selloff. Falling dominance during a rally is what traders call alt season.
It is a ratio, so it can rise while everything falls. It is used for sector-rotation-style decisions within crypto.
Example: dominance moves from 52% to 58% over a quarter while total crypto market cap is flat. Altcoins as a group lost about 12% of their share.
Related: altcoin, halving, market-cap, sector-rotation